1. Field of the Invention
The present invention relates to a profile strip comprising a closure element and lateral webs extending from a base of the closure element, the lateral webs having attached thereto an adhesive layer of a different material than the lateral webs. More particularly, the invention relates to a polyolefin profile strip comprising the lateral webs which is co-extruded with the adhesive layer and to a method of forming a profile strip which may be attached to a package made of a material which is not compatible with the material used to make the lateral webs.
2. Description of Related Art
In the field of packaging, particularly in the area of food packaging, it is often desirable to package items in a package which may be repeatedly opened and closed. For example, when dealing with packaged foods, such as sliced luncheon meats, the consumer may initially use only a portion of the meats contained in one package. By providing a package which may be reclosed by the consumer, the consumer avoids having to locate a storage container for the unused portion of the food in the package. It will be appreciated that reclosable packaging appreciably enhances the marketability of such products.
A typical reclosable package is hermetically sealed by any suitable means, e.g., by a peel seal, a cut-off top, or a combination of the peel seal and a perforation top, which protects the integrity of the package until the consumer initially opens the package. By hermetically sealing a reclosable zipper to the package opening, the consumer may manually reseal the package after its use. A method for hermetically sealing such a reclosable zipper to a package web is disclosed in a copending U.S. patent application of Lawrence W. Wirth, Wayne M. Wegner, Mladomir Tomic, Raymond Buchko and Hans Natterer, Ser. No. 07/342,257, now U.S. Pat. No. 5,067,822, incorporated herein by reference. A reclosable means may also be used in combination with a peel seal.
The reclosable means may include a pressure fastenable seal in the form of a rib or male member located on one package wall and a mating groove or female member located on an opposing package wall. One particularly novel reclosable means is disclosed and claimed in a copending U.S. patent application of Steven H. Simonsen, Mladomir Tomic and Wayne M. Wegner, Ser. No. 07/354,141, filed on May 19, 1989, now U.S. Pat. No. 5,017,021, incorporated herein by reference.
If a package contains a peel seal, after the peel seal has been broken by the consumer, the package can be closed again by aligning the mating groove with the rib and forcing the two into a mating connection. Such a pressure fastenable seal may be easily broken to open the package. This is done by applying forces to separate the rib and the mating groove. By this method, the package may be repeatedly opened and closed by the consumer.
To facilitate the manufacturing of packages employing a reclosable seal, the reclosable seal may be provided in the form of a continuous profile strip which may be manufactured independently from the package itself. Such a profile strip is typically wrapped onto a spool or reel for storage until it is needed to complete the packaging process.
When a product is packaged, the profile strip is unwound from its storage spool and attached to the packaging substrate during the packaging process. Polyethylene sandwich bags provide a good example of the use of a profile strip (which includes a reclosable closure, or "zipper") which is attached to the bag. The profile strip is attached to the bag by heating a flange (also known as a lateral web) on the profile strip and the edge of the bag simultaneously. Because both materials are the same, e.g., polyethylene, they essentially melt together, forming an excellent seal; thus, they are compatible. As used herein, the term "compatible" means that two materials can be effectively heat-sealed together with the application of heat and/or pressure, but substantially without the use of a third material, e.g., an adhesive.
While polyethylene bags having a reclosable zipper are well known in the art, the use of a reclosable zipper in other packaging applications is also desirable in many situations. For example, it has been found that packaging of foods, such as cookies, potato chips, and sandwich meats, can be made more convenient for the consumer if a reclosable zipper is included in the packaging. This is done by employing a profile strip, which includes a reclosable zipper, compatible with the package, and heat sealing the profile strip to one edge of the package.
By way of example, food packages, particularly packages used in the packaging of meats, may be made from an ionomer resin, such as is marketed by DuPont under the trade name SURLYN.RTM.. Multi-layered films may also be used in such a packaging application, with a layer of SURLYN.RTM. either coated, laminated or co-extruded on the side of the film which will be used to heat seal the front and back of the package together with a hermetic seal.
When it is desired to include a reclosable zipper on a package, such as one made of SURLYN.RTM., it is not uncommon to make the profile strip which employs the zipper from the same material as the package. One reason for choosing the same material for the profile strip is that it will be compatible with the packaging film. Thus, the potential for an unsatisfactory heat seal is reduced by taking advantage of the heat sealing compatibility resulting from heat sealing together a profile strip and a package made of the same material.
However, one frequently occurring disadvantage to the use of the same material in the manufacture of the profile strip as is used for the packaging film (also referred to herein as a "package liner" or a "packaging web"), is that a substantial volume of material is required in the production of the profile strip due to the configuration of the profile strip. For example, when using a material such as SURLYN.RTM., which is relatively expensive, the manufacture of an all-SURLYN.RTM. reclosable package may be prohibitively expensive for many packaging applications.
Using a different, but cheaper, material for the profile strip would necessarily require that material compatibility would not be maintained, thereby giving rise to problems in obtaining a hermetic seal between the profile strip and the package liner, or heat sealable material. Using a cheaper material for both the package and the profile liner is also not a practical solution because of the design limitations placed on the material choice for use in the package.
For example, some materials, such as certain grades of SURLYN.RTM., may be used when packaging food products because they are FDA-approved for food contact. Also, SURLYN.RTM. is especially advantageous in a meat packaging application because it will seal through grease and other contaminants, e.g., moisture. Other, cheaper packaging materials may not share these benefits. Thus, whether a package liner will be compatible with a low-cost profile strip usually does not influence the decision of what kind of package liner will be used.
In addition to issues of cost, there are also other disadvantages which arise when attempts are made to employ a profile strip made from the same material as the package liner, depending on the particular material considered. To use SURLYN.RTM., as an example, one disadvantage which results from the use of SURLYN.RTM. profile strips is that SURLYN.RTM. tends to post-crystallize over time. As explained above, after the profile strips are formed, they are wound onto spools, which enables the profile strips to be stored in a continuous form until they are used in the packaging process. Unless the SURLYN.RTM. profile strips are used shortly after they ar made, the SURLYN.RTM. may begin to post-crystallize (also referred to herein simply as "crystallize") on the spool.
Crystallization of the SURLYN.RTM. profile strips causes the strips to harden. In addition, crystallization also causes the profile strips to shrink. Additional shrinkage results as the profile strips cool on the spools. The effect of the shrinkage and hardening of the profile strips is that they become tightly wrapped around the core of the spools. Thus, when the SURLYN.RTM. strips are removed from the spools, they may have a significant amount of camber--plastic deformation corresponding to the curvature of the spools.
Because the profile strips must be substantially flat when attached to the package on which they are to be used, the camber must be removed from the profile strips before they may be heat sealed to a packaging film. One method which has been used to straighten out the profile strips is to apply a substantial amount of tension to the profile strips prior to their attachment to the package.
Disadvantageously, the conventional equipment used in the packaging process is ill-suited for imparting such tensile forces to the profile strips. In addition, in some instances it may be impossible to remove the camber from the profile strips without destroying the strips. It will be appreciated by those skilled in the art that if a separate manufacturing process had to be employed to remove deformation from the profile strip, the cost of the packaging process would be significantly increased. Indeed, in some instances, the cost increase would be prohibitively high, causing the manufacturer to abandon the deformed profile strip. Additionally, some profile strips may deform and harden to such an extent that it may be impossible to remove deformation therefrom, and, as a result, they, too, may have to be discarded. Thus, it can be seen that there are serious disadvantages associated with using a SURLYN.RTM. profile strip to be used as a reclosable closure for a package also made of SURLYN.RTM..
Although the problems of deformation on the spools appear to be unique to SURLYN.RTM. packaging, high cost remains an issue with materials other than SURLYN.RTM. which are popular for use with food products because they have FDA approval. Thus, the primary disadvantage in employing a reclosable closure on food packaging is that in order to achieve an acceptable heat seal of the profile strip to the package, the profile strip must be made of the same expensive material used for making the package, or used for making the seal layer of the packaging web.
It has been suggested to use a polyolefin material, such as polyethylene, to make a profile strip for such an application. Polyethylene is substantially less expensive than most materials used as a seal layer for packaging. However, as indicated previously, a primary obstacle in the use of a polyethylene profile strip with such packaging is that polyethylene is not compatible with SURLYN.RTM. and other similar materials which are commonly used for making packaging webs, especially packaging webs for packaging foods.
It has been proposed to coat a polyethylene profile strip with an adhesive that would enable the polyethylene to seal to the packaging film. Coating, however, gives rise to several additional problems.
The method of coating is inconsistent. Coating tends to "spot," resulting in portions of the profile strip without adhesive, thereby causing a defective seal to occur. Additionally, some portions of the adhesive may be thicker than others, resulting in an inconsistency of the thickness of the resulting profile strip.
One significant limitation of coating is that only a thin layer of coating may be applied at a time. Several passes through the coating process may be necessary to build up the desired amount of adhesive. Thus, while coating itself requires an extra step in the manufacturing process, several extra steps may be required if the profile strips must be repeatedly coated to obtain a desired thickness of adhesive.
A more significant disadvantage to the use of a coating process is that many of the adhesives which are capable of being coated contain solvents. When used in an application involving food, FDA regulations may not allow the use of these adhesives because of the tendency of the solvents, which have a low molecular weight, to migrate through the package film and into the food.
Tilman, U.S. Pat. No. 4,354,541, discloses a closure strip of plastics material for use in manufacturing a resealable bag. The closure strip has a longitudinally extending profiled closure element upstanding with respect to one face of a ribbon-like base element and an opposite face surface adapted to be adhesively attached to the bag material. The opposite face surface includes adhesive-containing narrow spacer rib means projecting from the surface about the same distance as the desired thickness of the layer of the adhesive. The rib means define an adhesive layer-receiving wide channel area several times as wide as the rib means, and the rib means is adapted to maintain the opposite face surface spaced the same distance as the thickness of the adhesive layer from the surface of the substrate (the bag) when pressure is applied for pressing the strip and the substrate together. The adhesive is applied to the channel area, usually in the form of a bead by an adhesive applicator system. The adhesive-receiving area may extend along most of the opposite face surface, or a portion thereof. In one embodiment, the adhesive-receiving area extends only along that portion of the opposite face surface which is not directly under the profiled closure element.
Herz, British Patent Application No. 2 080 412, discloses a fastener strip having a profiled portion extending lengthwise, with an at least one face of the strip having a zone extending generally parallel to the profiled portion. The zone is at least partially covered with a layer of a heat-reactivatable adhesive. The adhesive may be deposited as a continuous layer or as a discontinuous layer, e.g., in the form of a plurality of bands separated by spaces without the adhesive. The adhesive is advantageously deposited continuously, at the outlet from the extrusion head producing the profiled connecting strips of the synthetic material. It has been found that the application of hot-melt adhesive to a previously formed profile strip in the manner of this patent produces an intermediate layer between the adhesive and the profile strip. Without wishing to be bound by any theory of operability, it is believed that this intermediate layer is formed of remelted and resolidified material from the profile strip. It has also been found that care must be taken in applying a hot-melt adhesive to a profile strip in the manner of this patent to avoid delamination between the profile strip and the adhesive after cooling.
Bentsen, U.S. Pat. No. 4,673,383, discloses a plastic separable fastener strip comprising a body having a base surface and a profile means projecting from the opposite end of the body than the base surface. The base surface is provided with a plurality of ribs having spaces therebetween. The principal consideration in determining the mass versus spacing relationship of the ribs is that the ribs can be fused without significant transfer of the fusion temperature to the fastener strip body. The ribs are made from the same material as the base surface.
Gould, U.S. Pat. No. 4,835,835, discloses a separable fastener profile strip made by a method comprising thermoplastically extruding the strip and providing it with a base surface, applying an adhesive in a fluent state to the base surface during extrusion of the strip, curing the extruded strip, and curing the adhesive on the base surface concurrently with curing of the strip. The fastener profile strip of Gould is free of lateral flanges.
However, in some industrial applications it is desirable, if not necessary, to produce a profile strip containing a closure element and lateral webs extending outwardly from base surface of the closure element. For example, vertical and horizontal form, fill, and seal machines are commonly used to partially form a package, fill the package with the package contents, and attach a profile strip to the package. When the profile strip is used in such machines, it is desirable to have a profile strip with two lateral webs extending from the opposite sides of the base of the strip to provide a means for handling and advancing the strip into engagement with the substrate. The lateral webs also enable the strip-guiding portion of the machines to steadily hold the strip and advance it into a precise alignment position with the substrate. The lateral webs also provide a convenient vehicle for carrying a layer of adhesive for attaching the strip to the substrate.
It is also advantageous in some industrial applications to co-extrude a layer of adhesive onto the lateral webs simultaneously with the extrusion of the strip containing the lateral webs. Co-extrusion reduces the cost of producing the strip because it integrates the adhesive-deposition step with the strip extrusion step, thereby eliminating a separate step in the manufacture of the strip. In addition, co-extrusion does not result in the formation of an intermediate layer between the adhesive and the profile strip, reducing the possibility of delamination.
Since the adhesive layer is different than the material used for forming the lateral webs, the adhesive layer has different physical properties, such as a coefficient of contraction. When the adhesive layer is co-extruded as a single, continuous layer with the lateral webs and the profile strip, subsequently to the co-extrusion step, the lateral webs, the profile strip and the adhesive layer are cooled to produce the finished profile strip having lateral webs containing the adhesive layer. However, the cooling step often results in deformation of the lateral webs. The deformation of the lateral webs causes operational problems since it is difficult to obtain a hermetic seal between the profile strip having deformed lateral webs and the substrate. The deformation also reduces the effectiveness of the lateral webs as a means of handling and guiding the strip since the deformed lateral webs do not provide a dependable means of guiding the strip, and, in fact, may cause the machines to malfunction.